The present invention relates to a flat heat exchanger for two gaseous media crossing each other, where one medium transfers heat to the other medium, such as the air entering and leaving a dwelling.
Flat heat exchangers of the type mentioned are used primarily in heat-recovery units in ventilation systems. An example is shown in the accompanying FIG. 1. The flat heat exchanger consists of a large number of laminations with spaces between them. Air entering and air leaving flow through alternate spaces. It is generally the heat from an airflow leaving the premises (exhaust air) which is transferred to an airflow entering the premises (makeup air), the air flows passing through the heat exchanger in different channels. The laminations are often made of aluminium and the distance between them can be maintained in various ways. One example is by means of ridges in the laminations.
Like all other types of heat exchangers, flat heat exchangers have both advantages and disadvantages. One of the greatest disadvantages with flat heat exchangers is the considerable risk of them freezing when the temperature outside drops below 0.degree. C. In recuperative heat exchangers the exhaust air is normally a warm, moist air and is cooled by a cold air flow consisting of fresh air or the like. These air flows exchange heat in the heat exchanger without coming into direct contact with each other. The cooling flow of fresh air or the like absorbs heat from the exhaust air, thus lowering its temperature. This causes precipitation or condensation of moisture on the heat-exchanging surfaces of the exhaust air channels in the system. When the outside temperature is low (below 0.degree. C.), this results in frost and the formation of ice. Such ice formation reduces the coefficient of heat transfer of the heat exchanger, leading to poorer heat transfer and necessitating a reduction in the temperature efficiency of the exchanger by by-passing a portion of the makeup air, for instance. A number of methods can be used to prevent ice forming and the exhaust air channels freezing up. A pressure gauge may be used, for instance, to sense when the pressure drop from the outflow side has increased due to ice, and the air entering can then be allowed to flow through the by-pass damper. However, it may take a considerable time for the ice to melt. Another method is to continuously regulate the by-pass damper so that ice is never formed. This can be achieved with the aid of a temperature transducer located where the air leaves the cold edge of the heat exchanger. All methods of preventing the formation of ice prevent maximum efficiency of the heat exchanger during the winter period. This is particularly noticeable in cold climates. All methods of preventing ice formation and freezing entail an extra loss of valuable energy.